Tag Archives: regex

PHP preg_match regex fail

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So this happened. Basically the first regex causes preg_match to fail when it tries to process 128KiB ASCII zeros… and 128KiB isn’t really that many zeros. Fortunately the second regex performs much better, PHP runs out of memory before preg_match chokes on that one.

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Sat Apr 01 13:17:17 [bash:5.1.16 jobs:0 error:0 time:243]
jj5@charm:/home/jj5/desktop/experiment
$ cat base64-regex.php 
<?php

test( '/^(?:[A-Za-z0-9+\/]{4})*(?:[A-Za-z0-9+\/]{2}==|[A-Za-z0-9+\/]{3}=|[A-Za-z0-9+\/]{4})$/' );
test( '/^[a-zA-Z0-9\/+]{2,}={0,2}$/' );

function test( $regex ) {

  echo "testing: $regex\n";

  $n = 0;

  for ( ;; ) {

    $n++;

    echo "n: $n\n";

    $string = str_repeat( '0', pow( 2, $n ) );

    $base64 = base64_encode( $string );

    if ( preg_match( $regex, $base64 ) ) { continue; }

    echo "error at N = $n.\n";

    return;

  }
}

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Sat Apr 01 13:17:21 [bash:5.1.16 jobs:0 error:0 time:247]
jj5@charm:/home/jj5/desktop/experiment
$ php base64-regex.php 
testing: /^(?:[A-Za-z0-9+\/]{4})*(?:[A-Za-z0-9+\/]{2}==|[A-Za-z0-9+\/]{3}=|[A-Za-z0-9+\/]{4})$/
n: 1
n: 2
n: 3
n: 4
n: 5
n: 6
n: 7
n: 8
n: 9
n: 10
n: 11
n: 12
n: 13
n: 14
n: 15
n: 16
n: 17
error at N = 17.
testing: /^[a-zA-Z0-9\/+]{2,}={0,2}$/
n: 1
n: 2
n: 3
n: 4
n: 5
n: 6
n: 7
n: 8
n: 9
n: 10
n: 11
n: 12
n: 13
n: 14
n: 15
n: 16
n: 17
n: 18
n: 19
n: 20
n: 21
n: 22
n: 23
n: 24
n: 25
n: 26
n: 27
n: 28
n: 29
n: 30
n: 31
n: 32
n: 33
n: 34
n: 35
Killed
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Sat Apr 01 13:18:21 [bash:5.1.16 jobs:0 error:137 time:307]

Regular Expressions (Perl-Compatible)

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I found the documentation for regular expressions at php.net.

I read somewhere on the internet to use the pattern modifiers “msU” to match multiline content, but while I understand the ‘m’ and the ‘s’ I don’t understand the ‘U’ very well. I’m not sure what it means to be greedy or ungreedy exactly. But I’ve been using msU pattern modifiers in my regexps that parse the phpjs.org content for import into jsphp.co.